We've had a number of interesting discussions about the disc manufacturing process in other threads, and while the quantity and diversity of information about plastic mold injection is steadily increasing here at DGR, it is scattered amongst numerous other topics. So I figured I would start a thread dedicated to the disc manufacturing process, to collect such information and discussion in a single place.

To start things off, I figured I would post this informative video about plastic injection molding that has been the dominant manufacturing process for discs ever since pie tins gave way to plastic...

I used to work in engineering and did a lot of work with plastic parts, including injection mold design and construction. I've also done some non-disc related work with the guys at Quest AT-- I don't know if they are even around anymore, but it was a great learning experience for me just to talk to the guy making molds and learning about the various issues he encountered. I could probably answer a lot of questions if anyone has any.

mikes919 wrote:I used to work in engineering and did a lot of work with plastic parts, including injection mold design and construction. I've also done some non-disc related work with the guys at Quest AT-- I don't know if they are even around anymore, but it was a great learning experience for me just to talk to the guy making molds and learning about the various issues he encountered. I could probably answer a lot of questions if anyone has any.

Cool! Here are a few questions, to begin:

1) What are the real kinds of plastics used for DX-type (incl. D, Pro-D, etc.), Pro-Type (incl. X, etc.), Champion-type (incl. Z, etc.), and Star-type (incl. ESP, etc.)? It seems that the early plastics were made from something common like some sort of blend of polyethylene or polypropylene, while the Z and later generation plastics contain polyurethane.

2) How do manufacturers account for plastic shrinkage when designing a mold? Do they use mathematical (e.g., finite element) models of the shrinkage, or do they just go by trial and error (and spending a lot of time and money at the machine shop)?

1. You're right. From my understanding, the cheaper DX discs are predominantly polyethylene and the Champion/Star discs are primarily Polyurethane. There is a continuum there, some plastics will have more of one than the other, or they might have some completely different portions like Hytrel or Nylon. But the two main ones are generally the basis for the blends, I believe. They are all proprietary blends and there are a million different additives they can use for weight, flexibility, color, and feel.

2. Generally for a specific plastic and temperature you will assume a constant shinkage rate around the whole mold. For example, I used to design parts in 3D and then blow them up by a set percentage, then use the blown up model to create a mold. There are exceptions, where you learn from experience that certain shapes might shrink more or less than the general rule. I imagine they learned a lot in the early runs at the disc companies.

Discs are so large that they have quite a bit of residual heat when they come out of the mold, and the final shape can be strongly influenced by the conditions under which they are cooled. I know for a while at gateway they were cooling discs in special racks to make sure they cooled to an extra-overstable shape. That's not generally practical, so that's one reason why you see so much variation from run to run of the same disc.

I think that Pro is pure Hytrel and Stars are a polyurethane/Hytrel mix. Champ, E and Z and from feel and looks probably Opto too are straight polyurethane if you don't count additives.

Flat shots need running on the center line of the tee and planting each step on the center line. Anhyzer needs running from rear right to front left with the plant step hitting the ground to the left of the line you're running on. Hyzer is the mirror of that.

That video, while interesting from a historical standpoint, is also absolutely horrible from a physics standpoint...

Yea it doesn't seem like he has the best understanding of flight when you listen to him talk, but there's no denying that those original Infernos and Raging Infernos were bombers. He definitely had something figured out for a minute there. And they did some runs of discs in the same plastic as the first run Gateway Spirits, which is the closest to the old opaque CE that I've seen.

Much of what mikes919 and JR said is very similar to what I've heard from a variety of other sources. Unless anyone has additional information, the corresponding plastics (not including additives) appear to be:

mikes919 wrote:...Generally for a specific plastic and temperature you will assume a constant shinkage rate around the whole mold. For example, I used to design parts in 3D and then blow them up by a set percentage, then use the blown up model to create a mold. There are exceptions, where you learn from experience that certain shapes might shrink more or less than the general rule.

Discs appear to be one of these exceptions, owing to their asymmetry, causing the wing to droop upon cooling, thus lowering the PLH (Parting Line Height). A couple of guys I talked to who used to mold discs for Innova showed me a Firebird that they let cool in the mold for ~30 minutes, so that its shape stayed very similar to that of the mold. The top was very flat, the PLH was very high, and the disc is so over-stable and hooks so strongly that even the biggest of arms can only throw it 250'. They also said that the dome of a disc is produced entirely during shrinkage.

What this information implies is that the mold itself must have a more over-stable shape than the desired shape of the disc it produces, since shrinkage lowers the PLH. Mechanically, this makes sense since the flight plate partly braces the upper rim, while the underside of the rim is free to shrink inward. The differential shrinkage at the top vs the bottom of the rim then determines the final PLH.

The degree of upper rim shrinkage should change depending on whether the flight plate gives way easily (e.g., if it domes up easily) or if it is more rigid. In other words, whether the flight plate is a strong or weak brace against radial contraction. I think this explains why the early Blizzards (with the bubbles in the flight plate) turned out to be so over-stable: the bubbles made the flight plate so flexible that it could no longer brace the upper part of the rim against shrinkage, causing the flight plate to dome up instead of resisting radial contraction, so that the upper vs lower rim differential shrinkage was more uniform, thus producing a higher PLH than would a stronger flight plate.

Also, different plastics will shrink to different degrees, because they have different thermal expansivity and elasticity. When I looked at thermal expansion coefficients, the typical value for polyethylene was significantly greater than that of polyurethane. Therefore shrinkage of DX-type discs will be greater than for Champion-type discs. Thus the under-stabilizing effects of shrinkage will be greater for DX than for Champion, which would explain why Champion plastic discs tend to be more over-stable than DX plastic versions from the same mold. Of course, thermal expansivity isn't the only property that matters, it is actually the temperature change upon cooling times the thermal expansivity, and the operating temperature of different plastics is also probably different. Also, the elasticity of the plastic also matters, as well as the glass transition temperature. The only way to sort out all these competing effects is to do some models of the shrinkage process.

mikes919 wrote:...I imagine they learned a lot in the early runs at the disc companies...

I recall throwing some of the first discs produced by small start ups (e.g., the Medusa from Skyquest), and they were often very under-stable (more than the designer probably intended). Perhaps this is because they forgot to consider the effect that uneven shrinkage has on the stability characteristics of the disc, particularly the lowering of PLH.

mikes919 wrote:Discs are so large that they have quite a bit of residual heat when they come out of the mold, and the final shape can be strongly influenced by the conditions under which they are cooled. I know for a while at gateway they were cooling discs in special racks to make sure they cooled to an extra-overstable shape. That's not generally practical, so that's one reason why you see so much variation from run to run of the same disc.

No doubt, while a faster rate of production is more cost-effective, it also gives rise to greater shrinkage artifacts and opens the door to stronger variations from run-to-run. The only sure-fire way to make a consistent disc is to allow it to anneal completely in the mold, but that costs a lot of money since the production rate drops significantly. These kinds of trade-offs could also be modeled and made quantitative, I wonder if any of the disc makers have done anything this sophisticated (such a thing is straightforward for a scientist or engineer).

Bamba explains that they use a plastics blending supplier, and they go back-and-forth with them to come up with blends that work with a given mold and process. Makes sense, it is easier and probably far cheaper to tweak the plastic than the mold, since the mold is the most expensive part of the process.

As an industrial designer, I design products and as disc golf nuts, I wish I could do my own disc ( the only thing missing is the cash to do the machined mold! I guess China might come to help there).

Here is some videos, I found on youtube about disc golf disc manufacturing:

Side note on the last video about Vibram manufacturing process, it's compression molding compare to injection molding.

Also, I work with a special technical plasctic often called UHMW for Ultra-high-molecular-weight polyethylene and I always wonder if we could use the ultra resistant plastic as it can resist abrasion (its self lubricate), resist high and low temps and can be formed like Vibram disc (compression molding).